Aromatic polyimides were well-known as polymeric materials of high performance for their excellent thermal stability and balanced mechanical and electric properties. Polyimides were mainly used in the aerospace and electronic industries in the forms of films and moldings. Other uses for these polymers such as adhesives, gas separation membranes, composite matrices, coatings, and foams were rapidly increasing. However, aromatic polyimides are difficult to be processed because of high softening temperatures and limited solubility in commercially available solvents; accordingly the research and improvement of their solubility or heat plasticity without decreasing their original excellent characteristics were the aims of the present scientists.
The literature is flooded with claims of polyimides according to our specific paper and patent search for soluble polyimides based on dianhydride. Most of the literature described the improvement of diamine in the poly(ether imide)s polymerization and relatively less literature described about the invention of dianhydrides in the poly(ether imide)s. The new dianhydrides used in improvement of soluble polyimides were usually cyclo-paraffin system, diphenylsulfone tetracarboxylic dianhydride (DSDA) system and 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) system. Polyimides with ortho-linkage aromatic groups and/or the diether dianhydride system containing the cardo pendant group could also improve the solubility. Among these soluble polyimides, up to the present, those with better effects were of 6FDA system. Polyimides with the paraffin ring system were poorer in thermal stability; polyimides with the DSDA system exhibited less solubility improvement; and polyimides with ortho-linkage aromatic groups and/or the diether dianhydride system containing the cardo pendant group were adversely affected in the degree of polymerization and easily decomposed by concentrated sulfuric acid.
Colorless or light-color PIs have an increasing demand in the applications of monitors, optical devices and fibers, etc. Basically, the aromatic PIs are deep-yellow or red-brown. Up to the present, the PIs of the 6FDA system and of the diether dianhydride system containing the cardo pendant group have better colorless property. However, the former are relatively expensive, and the latter as described above are poor in the degree of polymerization, mechanical and thermal properties. Thus, a lot of effort and attempts have been made to improve the processing and colorless properties of the aromatic polyimides.
Our search results for soluble polyimides based on dianhydride in literature and patent are listed as follows:
(a) Patent list PA1 (b) Paper list
JP 03,243,629(1990); Chemical Abstract 116: 130366y PA2 JP 01 22,963(1989); Chemical Abstract 111: 155040x PA2 JP 64 00,121(1989); Chemical Abstract 111: 58573r PA2 JP 61,174,974(1986); Chemical Abstract 106: 103939z PA2 JP 61, 171,762(1986); Chemical Abstract 106: 34788k PA2 JP 59,199,720(1984); Chemical Abstract 102: 114157m PA2 Koo S.-Y; etc., J. Appl. Polym. Sci. 61(7), 1197(1996). PA2 Bryant, R. G., High Perform. Polym. 8(4), 607(1996). PA2 Young, P. R.; etc., J. Polym. Sci., Part A: Polym. Chem. 28(11), 3107(1990). PA2 Omote, T.; etc., J. Appl. Polym. Sci. 38(3), 389(1989). PA2 Kusama, M.; etc., Tokyo Kogei Daigaku Kogakubu Kiyo 11, 49(1988). PA2 Eastmond, G. C.; etc., Polymer, 34, 2865(1994); 35, 5418(1994). PA2 Eastmond, G. C.; etc., Macromolecules, 28, 2140(1995). PA2 Hsiao S.-H.; Yang C.-P.; Chu K.-Y., Macromolecules, 30, 165(1997). PA2 Hsiao S.-H.; Yang C.-P.; Chu K.-Y., Macromol. Chem. Phys. 198, 2153(1997).